It is proposed to investigate the biological, biochemical (molecular biological) and immunological properties of the many members of the family of murine mammary tumor viruses (MuMTVs). Each strain of mouse seems to carry at least one and possibly two strain-specific MuMTVs. The virulence of these viruses varies greatly and usually depends on the test strain of mouse. There are differences in the polyacrylamide gel electrophoretic patterns of some of these viruses. The function of these differences will be studied. Infectivity and replicative characteristics of these various viruses will be studied in cultured cells of many animal species and also in many mouse strains.